The DNA of eukaryotes is packaged with core histone octamers to form nucleosomal arrays. In certain regions of the genome (e.g., telomeres), nucleosomal arrays are assembled into specialized structures termed heterochromatin. Genes packaged within heterochromatin tend to be transcriptionally silenced, distinguishing them from the genes which either are regulated or constitutively expressed. In S. cerevisiae, telomeric heterochromatin is associated with formation of a specific heritable nucleoprotein structure assembled from nucleosomal arrays and the silent information regulator (Sir) proteins, Sir2p, Sir3p, and Sir4p. It has been proposed that Sir proteins function in a multi-subunit SIR complex to help create a transcriptionally silenced heterochromatin domain. In the present proposal, I plan to use quantitative biochemical and biophysical approaches to characterize the macromolecular interactions of Sir proteins, both in the absence and presence of nucleosomal arrays. Specifically, I propose to: (1) characterize the self-association and heteromeric interactions of yeast telomeric silencing proteins in solution in vitro, (2) characterize the interaction of yeast telomeric silencing proteins with nucleosomal arrays in solution in vitro, (3) characterize the structural features of the supramolecular nucleoprotein complexes assembled in vitro from yeast silencing proteins and nucleosomal arrays using analytical hydrodynamic, electrophoretic, and electron microscopic techniques. The proposed studies will be the first to characterize the assembly, stability and higher order structures of the nucleoprotein complexes formed in vitro from defined nucleosomal arrays and pure yeast telomeric silencing proteins. The information obtained from these studies will provide a framework for understanding how chromatin fibers are assembled into functionally specialized heterochromatin domains. Additionally, the innovative approach developed to quantitatively study telomeric silencing protein complexes will be generally applicable for rigorous solution analysis of macromolecular assemblages in the giga-dalton size range.